The Delian league built a strong.

A. navy

B. army

C. fortress

D. highway system

The Military and Financial Success of the Delian League

The Athenian-dominated Delian League enjoyed success after success against the Persians in the 470s and 460s.

The Delian League (or Athenian League) was an alliance of Greek city-states led by Athens and formed in 478 BCE to liberate eastern Greek cities from Persian rule and as a defence to possible revenge attacks from Persia following the Greek victories at Marathon, Salamis, and Plataea in the early 5th century BCE.

I think that the answer is B. army